Project Summary The goal of the proposed study is to identify and determine the function of circular RNAs (circRNAs) in innate immunity. Appropriate activation of the immune response is crucial for host fitness. While effective control of invading pathogens depends on rapid and robust induction of immunity signaling pathways, prolonged or aberrant activation, either systemically or locally, can lead to pathological conditions in humans such as autoimmunity and cancer. Thus both the magnitude and duration of innate immunity signaling need to be tightly regulated. Non-coding RNAs (ncRNAs), exemplified by small interfering RNAs (siRNAs) and microRNAs (miRNAs), play a critical role in modulating antiviral innate immunity in a wide variety of organisms. Recent advances in deep- sequencing technology and computational biology have greatly expanded the repertoire of regulatory ncRNAs, with circRNAs as the latest addition. Besides their unique configuration, circRNAs are also distinct from their canonical linear RNA siblings by harboring frequent exon scrambling events. Originally viewed as merely by-products of rare ?head-to-tail? splicing events, circRNAs have recently been characterized as an abundant class of RNAs in nematodes, flies, mice and humans. With the exception of only a handful of circRNAs, the function of the vast majority of circRNAs is unknown. We identified a collection of Drosophila circRNAs that are differentially expressed in response to bacterial infection, and our preliminary studies have implicated select circRNAs in modulating innate immunity. We hypothesize that select circRNAs function as modulators of innate immunity signaling, and propose to employ a combination of genomic, computational, genetic, and biochemical approaches to comprehensively discover these enigmatic RNA species, and establish their function in the context of innate immunity signaling. Completion of the proposed study will establish circRNAs as a novel class of regulatory ncRNAs and uncover their function in modulating innate immunity. This will advance our understanding of their evolutionarily conserved mammalian counterparts, and provide insights into the molecular mechanisms underlying innate immunity signaling and ncRNA function.